Toxic and heavy metals are one of the most problematic classes of contaminants due to their ubiquity and toxicity. Heavy metals represent a significant source of pollution when released into the environment. They are present in fossil fuels and ores, for example, and are released into the environment via airborne emissions during industrial processing of these materials, e.g., during incineration, or leach into soils and groundwater from ash and other residues when these materials are landfilled. Heavy metals from all sources present a major environmental concern.
Heavy metals are present in fossil fuels such as coal, oil and natural gas, in biomass, in ores and in wastes. Heavy metals are volatilized in the hot regions of process units such as boilers, incinerators or furnaces used for waste disposal, energy generation or metal recovery. Subsequently, as the gases are cooled, less volatile metal species (e.g., cadmium and lead) condense onto particles of ash entrained in the gas stream, while more volatile metals (e.g., arsenic and mercury) remain in the gas phase, where they end up as airborne emissions.
Heavy metals include, for example, arsenic, beryllium, lead, cadmium, chromium, nickel, zinc, mercury and barium. Most of these metals are highly toxic to humans and animals. Metal-contaminated wastes often also contain organic contaminants. Thus, treatment technologies for treating wastes contaminated with toxic metals preferably should be effective for treating organic waste as well.
The necessity of dealing with toxic metals has given rise to important and growing technologies devoted to remediating soils, wastes, and other materials contaminated with them, and preventing their escape into the environment. Wastes contaminated with toxic metals and/or organics have been treated, for example, by immobilizing the waste materials or by incineration. Many treatment techniques use cement-forming additives or polymer resins to encapsulate the waste, and subsequently landfill the resulting mass. This technique is costly and may not destroy organics in the waste. Other treatment techniques melt the waste in conjunction with glass-forming materials into a glassy form. These techniques are even more expensive and energy intensive because of the very high temperatures required, and also may result in airborne emissions. Incineration is a more cost-effective treatment for waste remediation as it destroys most organic materials. However, incineration generates airborne emissions and/or residual ash streams containing high concentrations of toxic metals in leachable form. The metal-containing ash then requires further treatment, generally immobilization, to render the metals non-leachable.
In U.S. Pat. No. 4,820,325, Wheeler describes a process for treating a filter cake containing toxic metals, particularly arsenic, which were filtered from a liquid waste stream. This process involves mixing the contaminated filter cake, which is formed of a glass precursor material, with another glass precursor material and heating the mixture to form a molten mass. The mass is allowed to cool, thereby forming a solid glass entrapping the toxic metals.
In U.S. Pat. No. 4,859,367, Davidovits describes a cementation process for immobilizing heavy metals by combining metal-contaminated waste with an alkali-activated silico-aluminate geopolymer binder to form a solid material.
In U.S. Pat. No. 4,941,772 Roesky et al. describe a cementation process for treating contaminated wastes and combustion residues, such as ash, which may contain toxic metals. The process involves mixing the wastes and ash with water and calcium oxide, compacting the mixture to form briquettes and hardening the briquettes in an autoclave with saturated steam.
In U.S. Pat. No. 4,855,082, Duivelaar describes a cementation process for treating chemical waste by immobilizing it in glass. The process involves mixing the waste with molten silica glass and cooling the resulting mixture to form a solid glass entrapping the toxic materials.
These methods suffer from several drawbacks. None of the above methods are completely effective or practical for remediating wastes containing toxic metals, particularly when the metals are associated with organic components. These methods encase the metals in a cementitious material which forms a physical barrier but do not render the metal chemically inert. Methods that involve the formation of a molten glass are very energy intensive. Most importantly, all of the previous methods do not separate the metals from other wastes, which requires that large amounts of the glass or other binder be used to encase nontoxic materials which, if separated from the metals, could be disposed conventionally at a much lower cost.
A cost-effective method of treating wastes and other materials contaminated with toxic metals is needed.